The present invention relates generally to coatings formed by plasma spray. More particularly, the present invention relates to a coating having dense columns separated by gaps.
Liquid carrier plasma spray techniques, such as suspension plasma spray and solution precursor plasma spray techniques, have been developed to produce coatings having generally vertically-oriented columnar structures, similar to those produced by electron beam-physical vapor deposition (EB-PVD), separated by inter-columnar gaps or cracks, which can increase tolerance to the effects of thermal cycling during operation. The columnar structures are formed during material deposition by preferential material accumulation on surface asperities on a substrate. The columnar structures are porous, having nano-size pores, and are separated by porosity bands or areas of low material accumulation, which can accommodate thermal expansion and contraction of the coating. While the columnar microstructures can improve strain tolerance, the intra-columnar porosity can compromise the durability and toughness of the coating. Prior art liquid carrier plasma spray techniques teach modifying spray parameters such as standoff distance (distance between the plasma spray torch and the substrate), to increase the coating density. However, increasing density in such manner can result in a more uniform deposition of particles on the substrate and the loss of strain-tolerant columnar structures formed by preferential material accumulation.